Yes you can. To start with gdm put these lines:
/usr/bin/killall synergyc
sleep 1
/usr/bin/synergyc w.x.y.z.
in the file /etc/X11/gdm/Init/Default right before "sysmodmap=/etc/X11/Xmodmap"
Next file you will have to create if you don't have one: /etc/X11/gdm/PostLogin/Default
/usr/bin ...